Atomic nuclei were created largely during the “big bang,” by subsequent fusion reactions between protons and neutrons in the interior of stars, and in supernova. When elements are formed, electrons are added to the lowest available quantum level in numbers equal to the number of protons in the nucleus. Electrons are added to the atoms in a distinct sequence, from lowest quantum level electrons to highest quantum level electrons. The relative quantum energy of each electron is shown in Figure 2.4.
Table 2.2 Quantum designations of electrons in the 92 naturally occurring elements. The numbers refer to the principal quantum region occupied by the electrons within the electron cloud; small case letters refer to the subshell occupied by the electrons.
| Principal quantum number | Subshell description | Number of electrons |
|---|---|---|
| 1 (K) | 1s | 2 |
| 2 (L) | 2s | 2 |
| 2p | 6 | |
| 3 (M) | 3s | 2 |
| 3p | 6 | |
| 3d | 10 | |
| 4 (N) | 4s | 2 |
| 4p | 6 | |
| 4d | 10 | |
| 4f | 14 | |
| 5 (O) | 5s | 2 |
| 5p | 6 | |
| 5d | 10 | |
| 5f | 14 | |
| 6 (P) | 6s | 2 |
| 6p | 6 | |
| 7 (Q) | 7s | 2 |
| Total = 92 | ||
Figure 2.4 The quantum properties of electrons in the 92 naturally occurring elements, listed with increasing quantum energy (E) from bottom to top; K‐shell electrons violet; L‐shell blue; M‐shell bluish green; N‐shell green; O‐shell yellow; P‐shell orange and Q‐shell red.
The diagonal rule is a simple rule or memory device that predicts, with very few exceptions, the sequence in which electrons are added to the electron cloud. The order in which electrons are added to shells is depicted by a series of diagonal lines from 1s to 7p (Figure 2.5).
Table 2.3 shows the ground state electron configurations for the elements. One can write the electron configuration of any element in a sequence from lowest to highest energy electrons. For example, calcium (Z = 20) possesses the electron configuration 1s2, 2s2, 2p6, 3s2, 3p6, 4s2. Elements with principal quantum levels (shells) or azimuthal quantum s‐ and p‐subshells that are completely filled (that is they contain the maximum number of electrons possible) possess very stable electron configurations. These elements include the noble gas elements such as helium (He), neon (Ne), argon (Ar), and krypton (Kr) which, because of their stable configurations, tend not to react with or bond to other elements. For elements other than helium, the highest quantum level stable configuration is s2, p6, sometimes referred to as the “stable octet. ”
Figure 2.5 The diagonal rule for determining the sequence in which electrons are added to the electron cloud; K‐shell electrons violet; L‐shell blue; M‐shell bluish green; N‐shell green; O‐shell yellow; P‐shell orange and Q‐shell red.
2.2 THE PERIODIC TABLE
The naturally occurring and synthetic elements discovered to date display certain periodic traits; that is, several elements with different atomic numbers display similar chemical behavior. Tables that attempt to portray the periodic behavior of the elements are called periodic tables. It is now well known that the periodic behavior of the elements is related to their electron configurations. In most modern periodic tables (Table 2.3) the elements are arranged in seven rows or periods and eighteen columns or groups. Two sets of elements, the lanthanides and the actinides, which belong to the sixth and seventh rows, respectively, are listed separately at the bottom of such tables to allow all the elements to be shown conveniently on a printed page of standard dimensions. For a rather different approach to organizing the elements in a periodic table for Earth scientists, readers are referred to Railsback (2003).
2.2.1 Rows (periods) on the periodic table
On the left‐hand side of the periodic table the row numbers 1–7 indicate the highest principle quantum level in which electrons occur in